This invention relates generally to the detection of cracks in the rotor of a fluid powered turbine and in particular to a new method for the detection of incipient rotor cracks while the turbine is on-line under normal load.
The rotor of a turbine (for example, the rotor of a large steam turbine of the type used in the commercial generation of electrical power) is subjected to frequent severe stress due to any number of fairly routine turbine operating conditions. Included among these are such things as changes in load and operating temperature. Although rotor forgings are designed to withstand these stressful conditions, and while they have an excellent record of safety and reliability, cracks are known to develop under some circumstances in some rotors following years of service. The development and growth of a crack is by no means predictable, however, and in some extreme cases steam turbine rotors have actually burst in a brittle fracture mode.
Obviously, a cracked rotor must be replaced or repaired to protect equipment and personnel and to ensure continued efficient operation. If a crack is discovered soon enough, while it is small, the rotor may be economically repaired and returned to service in a relatively short time. On the other hand, if the crack has grown to the point that the rotor must be replaced, there is not only the high cost of replacement but even greater cost resulting from the loss of power generating capacity. Thus, there has been a nearly constant search in the art for improved methods and apparatus by which an incipient crack can readily be detected in a turbine rotor so that corrective action can be quickly taken.
A number of techniques are presently known and used to detect the presence and growth of a rotor crack. These include (1) surface inspection methods such as magnetic particle testing, eddy current testing, and dye penetrant techniques; and (2) volumetric methods such as ultrasonic testing (audiography). Unfortunately, none of these techniques is suitable for inspection while the machine is on line running under load. Operating personnel therefore try to ensure themselves against a major loss by scheduling periodic outages during which the turbine is inspected and tested by such techniques. While these periodic inspections provide an assessment of the condition of a rotor as it is at the time the investigation is performed, there is always the risk that a crack might initiate and grow between inspections. The technique of vibration signature analysis has also been used for rotor crack detection and has offered some relief from the need to bring the machine to a complete stop, but heretofore this technique has been useful only by taking the machine off line and then decelerating it to nearly zero speed to determine if a crack is present.
Accordingly, it is a general objective of the present invention to provide a rotor crack detection method which is useful for detecting rotor cracks while the turbine is on line operating under normal load and early enough to permit the rotor to be repaired without extensive down time.
Further objectives and advantages will be apparent from the ensuing description of the invention, its principles, and operation.